This invention relates to power supply circuits for supplying high voltages to loads, such as neon signs and the like, and more particularly, to a power supply circuit which automatically interrupts the supply of the high voltage to a load under certain fault conditions.
Certain functional devices require operating voltages in the order of several hundred to several thousand volts. For example, neon tubes used for signs require such high voltages for stimulating inert gases contained within the tubes to produce visible light. Power supply circuits for neon signs generally include a step-up transformer having a primary winding connected to line voltage at 120 VAC and a secondary winding to which the neon tube is connected. The step-up transformer transforms the line voltage at 120 VAC to the high voltage required for lighting the neon tube.
Under certain fault conditions, the presence of the high voltage on the secondary winding of the transformer presents a potentially dangerous condition. For example, when tubing of a neon sign is broken, or when the neon sign is disconnected from the output of the power supply circuit, either purposefully or inadvertently, the power terminals may be exposed, presenting a potential shock hazard.
Accordingly, power supply circuits for neon tubes have been proposed which reduce the magnitude of the voltage provided on the secondary winding of the power transformer under certain fault conditions. Such power supply circuits include a sensing circuit which senses if the secondary winding is loaded or not and reduces power to the load whenever an abnormal condition is sensed. One neon tube power supply circuit includes a transformer which is connected in circuit with the power step-up transformer in the output stage of the power supply circuit. The sensing circuit responds to a voltage level that occurs on the primary side of the power supply transformer as the result of the secondary load being removed to enable a protective circuit to prevent a transistor of a high voltage output stage from being over driven. One disadvantage of this arrangement is that the sensing circuit is connected directly in the high power output stage of the power supply circuit. In addition, a separate transformer is required for sensing the fault condition, adding considerable cost to the power supply circuit.
In another known power supply circuit, the power transformer includes a sensing winding which couples a voltage indicative of a no load condition to a control circuit. The control circuit responsively causes the power supply circuit to be placed in a stand-by mode for as long as the no load condition exists. A serious shortcoming of this arrangement is that if someone were to contact the secondary electrodes while the power supply circuit is in the standby mode, such contact could be sensed as load being reconnected and the high voltage would be reapplied to the secondary winding, allowing a shock hazard to exist.